I don’t think it will make much difference. I have made models where there is a lot of room in the chest and they run just as well as ones where the fit is extreamly tight to the point of having to cut clearance notches into the sides of the valve so the air/steam has room to get to both inlet ports.

Slightly different with steam particularly on a compound where the volume can act as a reciever.

If you just want the model to run under no load then it doesn’t really matter.

But from an engineering point of view it does matter. If the steam chest volume is small then as the steam starts to enter the cylinder steam will be drawn from the boiler via the steam passages. Inevitably there will be a pressure drop right from the start of admission. If the steam chest has some volume then the pressure in the steam chest has a chance to get close to boiler pressure while the steam in the cylinder is undergoing expansion and the pressure drop during admission will be less. A starting point might be to make the steam chest volume 30 to 40% of the swept volume.

As Jason says things become more complex with compound engines. In theory with a Woolf expansion engine, where the HP and LP pistons are in sync, then steam can flow direct from the HP exhaust to the LP admission. When I modelled the steam flows for my single crank compound traction engine (which is basically a Woolf arrangement) I found that losses were quite high unless the LP steam chest had a significant volume. If the volume is too small the back pressure can become significant.

For compound engines where the pistons are not in phase then a large volume is needed to store the steam until the LP cylinder is ready for it. Marine triple and quad expansion engines sometimes had seperate receivers for the steam rather than just making the steam chests bigger.

Andrew